Z-propagating waveguide laser and amplifier device in rare-earth-doped LiNbO3

ABSTRACT

A rare-earth-doped waveguide device which exhibits stable cw laser and amplifier operation for near-infrared optical pumping in a room-temperature environment is provided. The waveguide device is comprised of an x- or y-cut LiNbO 3  substrate on which metal-diffused channel optical waveguides are formed parallel to, or nearly parallel to, the crystallographic z-axis. The LiNbO 3  substrate is rare-earth doped either by thermal diffusion of single or multiple rare-earth ions. Alternatively, the rare-earth doped substrate is doped with rare-earth ions during the growth of the crystal from which the substrate was prepared with additional thermal diffusion of rare-earth dopants as required. This orientation of the waveguide channel substantially parallel to the crystallographic z-axis permits reliable laser and amplifier action without the destabilizing effects of photorefractive optical damage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of provisional patentapplication Ser. No. 60/023,581 filed on Aug. 19, 1996, entitled"Z-Propagating Waveguide Lasers in Rare-Earth-Doped LiNbO₃ ".

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to rare-earth-doped LiNbO₃ laser devicesand, more particularly, it relates to z-propagating and nearz-propagating waveguide laser devices in rare-earth doped LiNbO₃ inwhich the optical waveguide is oriented parallel or nearly parallel tothe crystallographic z-axis of the LiNbO₃.

2. Description of the Prior Art

The success of fiber amplifiers and lasers has recently stimulated agreat deal of interest in rare-earth-doped planar waveguide devices forproviding signal-processing functions on a local scale both in opticalcommunications and sensor systems. In particular, rare-earth-dopedLiNbO₃ is extremely attractive for signal processing functions since therare-earth-doped LiNbO₃ potentially permits a high degree of integrationthrough a combination of the existing mature waveguide fabricationtechniques, the intrinsically good material properties in therare-earth-doped LiNbO₃, and the optical gain produced by the rare-earthion dopants. Moreover, the incorporation of rare-earth ions in theLiNbO₃ crystals by indiffusion demonstrates a degree of versatility notreadily available in bulk rare-earth-doped planar waveguide devices.

Numerous integrated laser and amplifier devices have been demonstratedin the past in Nd- and Er-diffused LiNbO₃. See J. Amin et al, Opt. Lett.19, 1541 (1994); H. Suche, Proceedings of the 7th European Conference onIntegrated Optics, session ThA4 (Delft, 1995), pg. 565. The most commonmethod of waveguide fabrication in rare-earth-diffused LiNbO₃ is byTi-indiffusion allowing for low propagation losses and maintaining thespectral characteristics of the rare-earth ions. However, an inherentproblem with Ti:LiNbO₃ guided wave devices is the devices' relativeinstability at visible and near-infrared wavelengths as a result ofphotorefractive damage induced by the high optical power densities inthese guides. This has limited the demonstration of cw room-temperatureoperation of Nd-doped devices almost exclusively to the case where thewaveguides were fabricated by the annealed proton exchange process inMgO:LiNbO₃.

Photorefractive damage has also been one of the main reasons that themajority of Er:Ti:LiNbO₃ lasers and amplifiers have been pumped at 1480nm. The only report of a 980 nm pumped Er:Ti:LiNbO₃ device is describedby Huang et al, Electron. Lett 32,215, 1996. It should be noted that thedevice described by Huang was only an amplifier. His work demonstratedno laser action. In the Huang et al reference, the detrimental effect ofphotorefractive damage on the amplifier gain was evident and it isunclear as to whether net gain was obtained in the device. It is widelyaccepted, however, that the photorefractive effect is due tophotogeneration of electrons through ionization of Fe²⁺ impurities inthe Fe³⁺ state, and the subsequent migration of these electrons alongthe z-axis (photovoltaic effect). As described by Becker et al, Appl.Phys. Lett. 47, 1024, 1995, trapping of the electrons, presumably inareas outside the waveguide, results in regions of space charge whichperturb the waveguide modes through the electro-optic effect. Ingeneral, waveguides are fabricated in LiNbO₃ with the propagationdirection primarily perpendicular to the crystalline z-axis, in order touse the highest electro-optic coefficient (r₃₃) for on-chip modulation.However, the space charge separation caused by the photovoltaic effectis on the order of the mode diameter, and therefore the associatedfields remain largely within the waveguide, causing serious perturbationto the guided modes.

As was first reported by Holman, Proc. SPIE 408, 14, 1983, one way ofconsiderably reducing the optical damage is by orienting the waveguidesuch that light is constrained to propagate substantially parallel tothe crystalline z-axis. In this way, the charge separation is then alongthe guide length, and therefore the overlap between the fieldsassociated with this separation and the optical mode is minimized. Adisadvantage for the Holman z-propagation scheme is that it only allowsfor convenient use of the r₂₂ electro-optic coefficient, which is lowerthan the commonly used r₃₃ coefficient by a factor of approximately 9.However, the voltage requirement for switching in a z-propagatingwaveguide structure can be optimally made to be less than 15 V.Moreover, the effect of temperature changes in this z-propagatingwaveguide orientation, where both TE and TM modes are ordinary modes,are likely to be less than other orientations as dictated by thetemperature-dependent Sellmeier dispersion equations and the associatedtemperature-dependent birefringence of the material. Also, because thez-propagating waveguide of the Holman reference does not supportextraordinary modes, measures do not have to be taken during fabricationto suppress outdiffusion of lithium and spurious extraordinary waveguidemodes which are known to arise from such lithium outdiffusion will notoccur in the present invention. The fabrication of the present inventionis therefore simpler. Even through the work of Holman illustrates theadvantage of reduced photorefractive instabilities in optical waveguideswhich are oriented parallel to the crystallographic z-axis in LiNbO₃,published work also exists which illustrates that in some instances thephotorefractive damage may be significant. For example, in the paper ofSanford and Robinson, Proceedings of the 6th IEEE InternationalSymposium on Applications of Ferroelectrics, 4 (1986), the authors showdata which clearly indicates that the z-propagating waveguide geometryin LiNbO₃ may exhibit serious polarization switching photorefractiveinstabilities. Furthermore, the same authors in a second paper,Proceeding of the SPIE, Vol. 704, 58 (1987), showed that thesepolarization switching artifacts may occur on the time scale ofmilliseconds. These polarization switching photorefractive artifactswere found in some cases to be so severe that upwards of 100% of theoptical power could be exchanged between TE and TM modes. Consequently,with the work of Holman in conjunction with the work of Suche, incombination with the work of Sanford, a person skilled in the art wouldconclude that the z-propagating geometry is by no means an a-prioriguaranteed success. Only demonstration of the fact that such a laserwill indeed function, as done by the inventors of the present invention,and reducing the device to practice, as described herein, is conclusiveevidence that such a laser can indeed by realized.

Thus, there is a need for a rare-earth-doped waveguide laser withimproved stability at visible and near-infrared wavelengths. There isalso a need for a rare-earth-doped waveguide laser with reducedphotorefractive damage induced by high power densities. There is still afurther need for an Er-doped waveguide laser and amplifier which caneffectively be pumped at γ_(p) =980 nm given that 980 nm pumping hasproven to be more effective than 1480 nm pumping for locally pumpedfiber amplifiers and the cost/mW of 980 nm pump diodes is currentlylower than that of the 1480 nm diodes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide rare-earth-doped, ormultiple rare-earth-doped, LiNbO₃ waveguide lasers which are configuredsuch that the guided optical modes propagate substantially parallel tothe crystalline z-axis and can be pumped at any wavelength (from thevisible to the near-infrared) as required by the rare-earth-dopant(s),and which produces laser action without concern for photorefractiveinstabilities which may be introduced by various visible ornear-infrared pump wavelengths, i.e., γ_(p) less than 1000 nm or suchphotorefractive instabilities which may result from the laser actionproduced by the present invention itself. The waveguide laser device ofthe present invention has improved stability at visible andnear-infrared wavelengths and reduced photorefractive damage, andimproved stability when operating as a laser or an optical amplifier.

The present invention is a waveguide device which may be reproduciblyfabricated and produces stable cw laser output in a room-temperatureenvironment. The waveguide device comprises a z-propagating structure oneither an x-cut or y-cut LiNbO₃ substrate having at least one opticalwaveguide placed substantially parallel to the crystallographic z-axis.Rare-earth ions are incorporated into the LiNbO₃ substrate by diffusionin undoped LiNbO₃ crystalline plates, or by incorporation into the bulkcrystal, from which such plates are cut, as the bulk crystal is grownthereby forming a rare-earth-doped substrate. If the rare-earth ions areincorporated into the LiNbO₃ plate by diffusion, the diffusion of therare-earth species may be distributed over the entire surface of theLiNbO₃ plate, incorporating singularly or in combination one or more ofthe dopant ion species Er³⁺, Nd³⁺, Yb³⁺, and Tm³⁺, or the rare-earthdopant species may be concentrated in localized portions of the LiNbO₃plate by selective diffusion of any required combination or geometricpattern as necessary to obtain doping over only selected areas asrequired by the design of the laser or amplifier in consideration.

At least one metal-diffused waveguide channel is incorporated into therare-earth-doped substrate. Each metal-diffused waveguide channel issubstantially parallel to the crystallographic z-axis of the LiNbO₃substrate with the waveguide channel and the rare-earth-doped substrateforming a rare-earth-doped z-propagating waveguide wherein therare-earth-doped z-propagating waveguide provides room-temperature laseroperation substantially free from photorefractive instability.

In an embodiment of the waveguide device of the present invention, therare-earth ions are selected from the group consisting of Er³⁺, Nd³⁺,Yb³⁺, and Tm³⁺. Preferably, the rare-earth ions are selected from one ormore of the group consisting of Er³⁺, Nd³⁺, Yb³⁺, and Tm³⁺. If more thanone rare-earth ion is used, the multiwavelength operation is possible bypumping at a single wavelength or two different wavelengths depending onwhich combination of rare-earth ions are selected, i.e., Er/Ybcombination is pumped at a single wavelength, near 980 nm for example,then the device will lase near 1550 nm due to the direct excitation ofthe Er³⁺ ions by the pump in addition to the excitation of the Er³⁺ ionsby means of energy transfer from the exited Yb³⁺ ions to the Er³⁺ ionswherein the Yb³⁺ ions were also excited by the pump. Alternatively, ifthe device is pumped around 945 nm, the Yb³⁺ ions alone will be excitedsuch that they will provide laser action near 1031 nm. Therefore, such alaser, or optical amplifier, pumped near either or both 980 nm and 945nm could simultaneously produce lasing or optical amplification at neareither or both 1550 nm and 1031 nm. Furthermore, the rare-earth ions arepreferably indiffused into the LiNbO₃ substrate or may have beenincorporated into the bulk LiNbO₃ crystal when it was grown.

In another embodiment of the waveguide device of the present invention,the LiNbO₃ substrate allows multiwavelength operation. Additionally,preferably, the metal channels are metals which increase the refractiveindex and form a waveguide with the metals being selected from the groupconsisting of Ti, Zn, Ni, and Cu. Also, the metal channels arepreferably diffused into the LiNbO₃ substrate.

In yet another embodiment of the waveguide device of the presentinvention, the rare-earth-doped LiNbO₃ substrate which supports anoptical waveguide that is oriented substantially parallel to thecrystallographic z-axis also has modulator structure in the vicinity of,or overlapping with, the waveguide. It is understood that therare-earth-doped LiNbO₃ substrate may be selected to be either an x-cutor y-cut crystal. The modulator structure could then be used for, eithersingularly or in combination, mode-locking, Q-switching, or frequencytuning of the waveguide laser device.

In still another embodiment of the waveguide device of the presentinvention, the waveguide device further comprises a TE-TM polarizationswitching device formed in the rare-earth doped LiNbO₃ substrate whichsupports optical waveguides oriented parallel, or nearly parallel, tothe crystallographic z-axis. Preferably, the switching device allowsQ-switching of the waveguide device.

In yet still another embodiment of the waveguide device of the presentinvention, the waveguide device further comprises a pump light wave orsuitable collection of pump light waves. The pump light waves arecoupled into the waveguide channel as guided modes and singularly ortogether provide an excitation source for the rare earth ions composedof one or more of the group consisting of Er³⁺, Nd³⁺, Yb³⁺, and Tm³⁺, Insome instances, the pump light may not be constrained as a guided modeand still suitably excite the rare earth ion(s). This may be the case ifthe pump light is directed at the waveguide channel from the side ratherthan the end face of the waveguide channel. The suitably-excitedrare-earth ions then enable laser action in the waveguide when theendfaces of the waveguide are provided with the proper reflectivity toenable optical feedback at the lasing wavelength, or wavelengths, ofinterest. Moreover, the present invention acts as an optical amplifierwhen, in addition to the injection of guided pump light waves into thewaveguide, signal light waves of the appropriate wavelengths are alsoinjected as guided modes into the waveguide and experience gain andamplification through interaction with the excited rare-earth ions.

The present invention is also a method of forming optical waveguidesoriented parallel or nearly parallel to the crystalline z-axis of therare-earth-doped substrate. The method comprises selecting an x-cut ory-cut LiNbO₃ sample substrate, depositing a film or films of one or morerare-earth metals from the group consisting of Er, Nd, Yb, and Tm, ontothe substrate and introducing these by thermal diffusion into the LiNbO₃sample substrate to produce dopants of Er³⁺, Nd³⁺, Yb³⁺, or Tm³⁺ in theLiNbO₃ sample substrate. The distribution of these rare-earth dopantsdiffused into the LiNbO₃ substrate may cover completely or partially thesurface of the LiNbO₃ substrate as required by the operation of theparticular laser or optical amplifier in question. Alternatively, Er³⁺,Nd³⁺, Yb³⁺, and Tm³⁺ may already have been incorporated into therare-earth-doped LiNbO₃ plates when the bulk crystal, from which theplates were cut, was grown. The optical waveguides are formed in therare-earth-doped LiNbO₃ plates by depositing, either singularly or aseries, of metal stripes which are oriented parallel or nearly parallelto the crystallographic z-axis. The metal stripes are subsequentlyincorporated into the LiNbO₃ by thermal diffusion.

In an embodiment of the method of the present invention, the methodfurther comprises positioning the LiNbO₃ sample on a Pt pad, positioningthe pad on an alumina pedestal, and placing the LiNbO₃ sample, pad, andpedestal into an electric furnace. Further creating a flowing oxygenatmosphere about the LiNbO₃ sample, pad, and pedestal as they are heatedat high temperature in the electric furnace.

In another embodiment of the method of the present invention, therare-earth ions are selected from the group consisting of Er³⁺, Nd³⁺,Yb³⁺, and Tm³⁺. Preferably, the rare-earth ions are selected from atleast two of the group consisting of Er³⁺, Nd³⁺, Yb³⁺, and Tm³⁺.Furthermore, the rare-earth ions are preferably indiffused into theLiNbO₃ substrate.

In yet another embodiment of the method of the present invention, thewaveguide channels are formed by the diffusion of metal stripes into therare-earth-doped LiNbO₃ which increases the refractive index of therare-earth doped LiNbO₃ in the areas where the diffused metal stripesare present. The waveguide-forming metals are selected either singularlyor multiply, from a group consisting of Ti, Zn, Ni, and Cu.

Further objects, features, and advantages of the present invention willbecome apparent from a consideration of the following description andthe appended claims when taken in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of illustrating the structure andorientation of the waveguide laser device in which the optical channelis oriented parallel or nearly parallel to the crystalline z-axis of therare-earth-doped LiNbO₃ substrate constructed in accordance with thepresent invention;

FIG. 2a is a graph illustrating the cw laser characteristics of anembodiment of the Er-doped waveguide laser device in which the opticalwaveguide is oriented parallel or nearly parallel to the crystallinez-axis of the Er-doped LiNbO₃ substrate with the waveguide laser beingpumped at or near 980 nm and constructed in accordance with the presentinvention;

FIG. 2b is a graph illustrating the laser spectrum of the embodimentEr-doped waveguide laser device constructed in accordance with thepresent invention in which the optical waveguide is oriented parallel ornearly parallel to the crystalline z-axis of the Er-doped LiNbO₃substrate;

FIG. 3a is a graph illustrating the cw laser characteristics of anembodiment of the Nd-doped waveguide laser device in which the opticalwaveguide is oriented parallel or nearly parallel to the crystallinez-axis of the Nd-doped LiNbO₃ substrate with the waveguide laser devicebeing pumped at or near 814 nm and constructed in accordance with thepresent invention;

FIG. 3b is a graph illustrating the laser spectrum of an embodiment ofthe Nd-doped waveguide laser device constructed in accordance with thepresent invention in which the optical waveguide is oriented parallel ornearly parallel to the crystalline z-axis of the Nd-doped LiNbO₃substrate;

FIG. 4a is a graph illustrating the cw laser characteristics of anembodiment of a waveguide laser device in which the rare-earth dopantions are a combination of Er³⁺ and Yb³⁺ and the optical waveguide isoriented parallel or nearly parallel to the crystalline z-axis of thecombined Er- and Yb-doped LiNbO₃ substrate. The lasing characteristicshown is from the Er³⁺ ion with the signal being emitted atapproximately 1531 nm. The device was pumped at or near 980 nm;

FIG. 4b is a graph illustrating the laser spectrum of the embodiment ofa waveguide laser device in which the rare-earth dopant ions are acombination of Er³⁺ and Yb³⁺ and the optical waveguide is orientedparallel or nearly parallel to the crystalline z-axis of the combinedEr- and Yb-doped LiNbO₃ substrate with the laser output near 1031 nmfrom the excited Yb³⁺ ions being pumped with a pump wavelength near 945nm, thereby illustrating that, in accordance with the present invention,lasing at multiple wavelengths due to multiple rare-earth dopants arepossible;

FIG. 5 is a plan view illustrating a modulator structure constructed inaccordance with the present invention that can phase-modulate,polarization modulate, or amplitude modulate guided waveguide modes inoptical waveguides fabricated parallel or nearly parallel to thecrystalline z-axis of an x-cut LiNbO₃ substrate; and

FIG. 6 is a plan view illustrating a modulator structure constructed inaccordance with the present invention that can phase-modulate,polarization modulate, or amplitude modulate guided waveguide modes inoptical waveguides fabricated parallel or nearly parallel to thecrystalline z-axis of a y-cut LiNbO₃ substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated in FIG. 1, the present invention is a z-propagating ornear z-propagating waveguide laser device, indicated generally at 10, inrare-earth-doped Ti:LiNbO₃, where the rare-earth dopants are composed ofEr, Nd, Yb, and Tm, either singularly or in combination, and the opticalwaveguide channel is placed parallel or nearly parallel to thecrystalline z-axis and formed by the diffusion of Ti. The diffusion ofother waveguide-forming dopants may be used as described above Thewaveguide device 10 is made "z-propagating" by configuring the waveguide12 to be oriented parallel or nearly parallel to the crystallographicz-axis of the substrate 14. The z-propagating geometry allows for theformation of a very efficient TE-TM polarization switching device. Itshould be noted that the waveguide device 10 can be formed on either anx-cut LiNbO₃ plate, as illustrated in FIG. 4a, or a y-cut LiNbO₃ plate,as illustrated in FIG. 4b. The fabrication of such a polarization switchdelivers a very efficient and easily realized means for Q-switching,mode-locking, or wavelength tuning the waveguide laser device 10.

Three representative functional examples for embodiments of the presentinvention are now presented. It should be noted that variations of theprocessing details presented will also be effective in producingfunctional laser devices within the scope of the present invention. Indescribing the construction of the waveguide device of the presentinvention, three samples, Sample A, Sample B, and Sample C, will bediscussed. Fabrication of the rare-earth-doped Ti:LiNbO₃ begins byselecting an x-cut or y-cut LiNbO₃ sample. Using e-beam techniques,approximately 8 nm of Nd is deposited on sample A and approximately 15nm of Er is deposited on sample B. The Nd³⁺ ions are then preferablydriven into Sample A by indiffusion at approximately 1100° C. over aperiod of approximately 240 hours, and the Er³⁺ ions are preferablyindiffused into Sample B at approximately 1100° C. over approximately144 hours. On Sample A, Ti stripes approximately 6 μm wide andapproximately 90 nm thick are delineated along the z-axis using standardphotolithography and vacuum evaporation techniques. A similar process isused on Sample B to form Ti stripes approximately 7 μm wide andapproximately 110 nm thick. The Ti is then diffused into both Sample Aand Sample B over a period of approximately 9 hours with Sample A beingdiffused at a temperature of approximately 1005° C. and Sample B beingdiffused at a temperature of approximately 1030° C. The rare earthdiffusions and the waveguide diffusions are preferably conducted in aceramic tube placed in an electric furnace. Both Sample A and Sample Bare next placed on a Pt pad, which in turn is placed on an aluminapedestal with the alumina pedestal placed in the ceramic tube. Oxygenruns through the ceramic tube placed in the electric furnace with a flowrate of 1 liter/minute. Finally, both samples are cut and end-polished,yielding waveguides with a range of different lengths with end-facesthat are polished substantially perpendicular to the z-axis.

Another x-cut or y-cut wafer of LiNbO₃ referred to as Sample C is alsowithin the scope of the present invention. Using e-beam techniques, astack of rare-earth ions, consisting of alternating layers of Er and Yb₂O₃ are deposited on Sample C. Each individual rare-earth ion layer inthe stack is approximately 2 nm thick with the total thickness of thestack being approximately 28 nm. The layers are then diffused into theLiNbO₃ substrate at approximately 1100° C., for a total of approximately360 hours. Ti stripes having a thickness of approximately 110 nm and awidth of approximately 7 μm are delineated on the LiNbO₃ substrate usingstandard photolithography. The Ti diffusion is conducted atapproximately 1030° C., for approximately 9 hours. All of the Tidiffusions are preferably in flowing oxygen, in an alumina tube placedin an electric furnace with the Sample C sitting on a Pt pad. Thefinished Sample C device yielded waveguides which were approximately 2cm long.

It should be noted that while certain variables in time, temperature,and thickness have been set forth above in construction of Sample A,Sample B, and Sample C, it is within the scope of the present inventionto use lesser or greater time, temperature, and thickness variables toyield similar results.

Sample A--Nd:Ti:LiNbO₃

A near-field analysis was performed on the guides on the Er:Ti:LiNbO₃device using a Nd:YLF laser operating near 1040 nm. At this wavelength,the waveguides were slightly double-moded, with the fundamental modediameters (1/e full width)5.2(±0.3) μm in width and 2.8(±0.15) μm indepth. The waveguide also supported two transverse modes atapproximately 800 nm. Transmission measurements made at 809 and 850 nmrevealed a coupling efficiency of 68% in Sample A. With an estimated 20mW coupled into the waveguide, a single-exponential fluorescence decaywas observed, with a 1/e lifetime of 89 μs. The acousto-optic (AO)modulator was then removed, and the lasing characteristics of a1.8-cm-long device were measured. The device lased in a stable, cwmanner at 1093.1 nm with the feedback provided by the 14% Fresnelreflectance from the polished endfaces. Both the pump and laser emissionwere TE polarized.

FIGS. 3a and 3b illustrate the lasing characteristics of Sample A, withFIG. 3b illustrating a laser spectrum. The output power indicated inFIGS. 3a and 3b is the total power from the pumped and unpumped end ofthe device. For the case of this particular example, the absorbed pumppower was 70% of that launched. The threshold for laser oscillation was68 mW of absorbed pump power, and the slope efficiency was 40%. Theinventors of the present invention were able to extract approximately 40mW from Sample A, limited only by the available pump power, without anydiscernible sign of photorefractive damage. Note that the performance ofthis laser is representative only for the case described. Attachingmirrors with various reflectivities to the end facets of the waveguidedevice will result in modified laser behavior in terms of the outputpower, the precise output wavelength, and the laser threshold.

Sample B--Er:Ti:LiNbO₃

Near-field analysis was carried out on the Er:Ti:LiNbO₃ device using a1.5 μm light-emitting diode (LED), revealing the 7-μm-wide Er:Ti:LiNbO₃waveguides to be single-moded at this wavelength, with 1/e modediameters of 7.9 μm×4.6 μm (width×depth). The guides supported threetransverse modes at 980 nm. Laser characteristics were measured in this2.9-cm-long device, with cw pumping from the Ti:Al₂ O₃ at 980 nm. Thepump mode was TE polarized. A mirror with a reflectivity of >99% at1530, and which transmitted 85% of the pump, was attached to the frontface of the device and fluorinated liquid provided index-matching. Atthe output end of the device, no mirror was attached, and Fresnelreflection from the polished end-face was used to complete the lasercavity. The device operated very stably, with the output TE polarized;FIGS. 2a and 2b illustrate the cw laser characteristics. In particular,FIG. 2b illustrates the laser output spectrum which occurs near 1531.4nm. For the particular reflectivities of the mirrors attached to the endfacets of the waveguide laser device, the lasing threshold wasapproximately 10.5 mW of absorbed pump power and the device exhibited aslope efficiency of 8.5%. Stable laser output at power of 1 mW near 1550nm was obtained. In general, it was possible to make the device lase byattaching to the end facets a wide selection of mirrors that had variousreflectivities. Furthermore, the laser would still operate if no mirrorswere attached and only the Fresnel reflection of the end facets providedthe optical feedback.

Sample C--Er:Yb:Ti:LiNbO₃

Near field analysis was carried out on the Er:Yb:Ti:LiNbO₃ device usinga 1550 nm LED revealing the waveguides to be single moded, with 1/e modeintensity diameters of approximately 7.9 (±0.4) μm×4.6 (±0.25) μm. Theguide supports three transverse modes at 980 nm. Laser characteristicswere measured in Sample C, with cw pumping at 980 nm, with a highreflector at the input end and a 95% reflector at the output end. Thedevice lased in a stable cw mode at approximately 1531.4 nm, with athreshold of approximately 45 mW of coupled pump power and a slopeefficiency of approximately 0.6%.

The laser characteristics of Sample C are best illustrated in FIG. 4a.The laser waveguide was pumped at or near 980 nm where the pump lightdirectly excited the Er³⁺ dopant ions and also excited the Yb³⁺ dopantions which in turn transferred their energy to the Er³⁺ dopant ions.Laser action near 1531 nm from the excited Er³⁺ ions then resulted. Thepresence of the Yb³⁺ ions thus enabled more efficient optical pumping ofthe E³⁺ lasing ions than would be possible if the Er³⁺ ions were thesole rare-earth dopant.

Mirrors with high reflectivity near 1060 nm were attached to thewaveguide laser end faces to promote lasing from the Yb³⁺ ions. Thelaser would then operate near 1031 nm which is the peak which themaximum gain in the Yb:LiNbO₃ emission spectrum when pumped near 945 nm.This is an illustration of selecting laser action from the Yb³⁺ dopantsalone, even with the Er³⁺ dopants present, by tuning the pump lightwavelength to a range where the Yb³⁺ is primarily sensitive. The laseroutput spectrum is illustrated in FIG. 4b. It is also possible that withappropriate choice of mirrors, the device can be made to lasesimultaneously near 1030 nm and near 1530 nm by pumping near 980 nm.

Conclusion

The z-propagating waveguide laser device of the present invention is astable, room-temperature operating laser fabricated by Ti-indiffusion inrare-earth-doped LiNbO₃. The z-propagation scheme has been utilized inconstructing the z-propagating waveguide laser device of the presentinvention thereby allowing effective curbing of the instabilitiesarising from photorefractive optical damage. During experimentationswith a z-propagating waveguide laser device constructed in accordancewith the present invention, a Nd:Ti:LiNbO₃ waveguide laser device lasedcontinuously using only the polished endfaces to provide feedback. Theabsorbed pump power at threshold was approximately 68 mW and the slopeefficiency was approximately 40%. A similar z-propagating Er:Ti:LiNbO₃waveguide laser device constructed in accordance with the presentinvention was made to lase by pumping at approximately 980 nm, with anabsorbed pump threshold of approximately 10.5 mW and a slope efficiencyof approximately 8.5%, obtained using a high reflector on the input faceand only the polished output face as the second mirror. Further yet, az-propagating Er/Yb-doped TiLiNbO₃ waveguide laser device constructed inaccordance with the present invention was made to lase by pumping atapproximately 945 nm with stable lasing at approximately 1031 nm at athreshold of approximately 120 mW of coupled pump power.

When pumped by a suitable light source, optical feedback is providedfrom the end-facets of the waveguide by attaching to the end-facetssuitable mirrors that enable laser action of the excited rare-earth ionsand do not impede or restrict the injection of pump light into thewaveguide device. The mirrors may be directly deposited on the waveguideend-facets by means of well-known vacuum evaporation techniques fordielectric thin films. Alternatively, the mirrors may be separatelyformed on thin transparent substrates of a suitable material andmechanically attached to the waveguide end-facets with optical adhesivesor clips.

FIGS. 5 and 6 illustrate plan views of a modulator structure that canphase-modulate, polarization modulate, or amplitude modulate guidedwaveguide modes in optical waveguides fabricated parallel or nearlyparallel to the crystalline z-axis of x-cut LiNbO₃ or y-cut LiNbO₃plates. Fabrication of a modulator structure on the waveguide laserdevice described in the present invention will enable greaterfunctionality by enabling mode-locking, of the waveguide laser,Q-switching of the waveguide laser, or separately controlling thepolarization of the waveguide laser and allow wavelength tuning of thewaveguide laser. Furthermore, all four of these functions, i.e.,mode-locking, Q-switching, polarization control, and wavelength tuning,can occur simultaneously or separately as required by the intended useof the present invention. The voltages V2 and V1 as indicated in FIGS. 5and 6 control the degree of phase modulation and TE-TM polarizationconversion. The TE pass polarizer illustrated in FIGS. 5 and 6 enablesQ-switching by means of providing amplitude modulation of the laserthrough polarization switching and therefore loss modulation of the TElasing mode. Additionally, the modulator structure may be used in such amanner that it will enable, either continuous or discrete, wavelengthtuning of the laser output of the rare-earth-doped LiNbO₃ waveguidelaser.

Furthermore, the waveguide laser device described in the presentinvention can be mode-locked, Q-switched or simultaneously mode-lockedand Q-switched by attaching a semiconductor saturable absorber to theend facet of the waveguide rather than, or in combination with, theelectrode structures described above.

The discovery and demonstration of a rare-earth doped LiNbO₃ waveguidelaser device, especially an Er:LiNbO₃ waveguide laser device, pumped atapproximately 980 nm is a very important result, in view of theinexpensive and readily available pump laser diodes at the 980 nmwavelength. The discovery also opens up many opportunities for advancedactive and passive circuits incorporating, for example, on-chipwavelength division multiplexers for independent pump and signalrouting.

The foregoing exemplary descriptions and the illustrative preferredembodiments of the present invention have been explained in the drawingsand described in detail, with varying modifications and alternativeembodiments being taught. While the invention has been so shown,described and illustrated, it should be understood by those skilled inthe art that equivalent changes in form and detail may be made thereinwithout departing from the true spirit and scope of the invention, andthat the scope of the present invention is to be limited only to theclaims except as precluded by the prior art. Moreover, the invention asdisclosed herein, may be suitably practiced in the absence of thespecific elements which are disclosed herein.

We claim:
 1. A waveguide device for operating as a stable opticalamplifier or a waveguide laser in a room-temperature environment, thewaveguide device comprising:a LiNbO₃ substrate having a crystallographicz-axis; rare-earth ions incorporated into the LiNbO₃ substrate, therare-earth ions and LiNbO₃ substrate forming a rare-earth-dopedsubstrate; at least one metal diffused waveguide channel incorporatedinto the rare-earth-doped substrate, each metal-diffused waveguidechannel being substantially parallel or nearly parallel to thecrystallographic z-axis of the LiNbO₃ substrate, the waveguide channeland the rare-earth-doped substrate forming a rare-earth-dopedz-propagating waveguide; end facets formed on the rare-earth-dopedz-propagating waveguide, the end facets substantially perpendicular tothe axis of the waveguide; and wherein the rare-earth-dopedz-propagating waveguide providing stable room-temperature operation asan optical amplifier or as a waveguide laser, both substantially freefrom photorefractive instability.
 2. The waveguide device of claim 1wherein the rare-earth ions are selected from one or more of the groupconsisting of Er³⁺, Nd³⁺, Yb³⁺, and Tm³⁺.
 3. The waveguide device ofclaim 1 wherein the rare-earth ions are indiffused into the LiNbO₃substrate.
 4. The waveguide device of claim 1 wherein the rare-earthions are incorporated into the LiNbO₃ substrate during formation of theLiNbO₃ substrate.
 5. The waveguide device of claim 1 wherein the metalchannels are metals which increase the refractive index and form awaveguide, the metals being selected from one or more of the groupconsisting of Ti, Zn, Ni, and Cu.
 6. The waveguide device of claim 1wherein the metal channels are thermally diffused into the LiNbO₃substrate thereby forming optical waveguides.
 7. The waveguide device ofclaim 1 wherein the z-propagating substrate has a modulator structureselected from the group consisting of an x-cut LiNbO₃ plate and a y-cutLiNbO₃ plate.
 8. The waveguide device of claim 1 and further comprisinga TE-TM polarization switching device formed on the z-propagating LiNbO₃substrate, the switching device allowing Q-switching, mode locking, orwavelength tuning of the waveguide device.
 9. The waveguide device ofclaim 1 and further comprising a suitably-generated pump light injectedinto the rare-earth-doped waveguide exciting the rare-earth ions andenabling cw laser action and amplification of the rare-earth-dopedwaveguide device.
 10. The waveguide device of claim 9 and furthercomprising mirrors mounted to the end facets of the waveguide deviceenabling laser action of the excited rare-earth ions free from impedingthe injection of the pump light into the waveguide device.
 11. Thewaveguide device of claim 1 and further comprising a modulator electrodestructure fabricated on the surface of the substrate nearinglyapproximate the waveguide, and further comprising means for providingsuitable switching voltages to the modulator electrodes.
 12. Thewaveguide device of claim 10 wherein the modulator structure enablesphase modulation and polarization switching of the guided optical modes.13. The waveguide device of claim 10 and further comprising anattenuator absorbing or scattering either the TE or TM guided modes. 14.The waveguide device of claim 10 and further comprising pump lightinjected into the rare-earth-doped waveguide, wherein the waveguidelaser simultaneously operates as a mode-locked and Q-switched laser oronly as a mode-locked laser.
 15. The waveguide device of claim 1 andfurther comprising a semiconductor saturable absorber connected to atleast one or both, as required, of the waveguide facets enabling thewaveguide laser to operate mode-locked, Q-switched, or simultaneouslymode-locked and Q-switched.
 16. The waveguide device of claim 9 andfurther comprising a guided signal light injected into the waveguide,the guided signal light having a predetermined wavelength to interactwith the excited rare-earth dopants such that the signal light isamplified by the excited rare-earth dopant whereupon exiting the outputface of the waveguide, the signal light is amplified to a greateroptical power than when the signal light was presented at the input faceof the waveguide device.
 17. The waveguide device of claim 9 wherein thedevice is pumped with single or multiple wavelengths and lasing, ineither cw, mode-locked, Q-switched, or combined mode-locked andQ-switched, at single or multiple wavelengths.
 18. The waveguide deviceof claim 16 wherein the pump light is pumped with single or multiplewavelengths producing amplification of single or multiple injectedsignal light waves at various wavelengths.
 19. The waveguide device ofclaim 9 and further comprising means for electro-optic tuning andadjustment of the output lasing wavelength by applying a suitablevoltage to an electrode position on or nearingly adjacent to thewaveguide channel or channels.
 20. The waveguide device of claim 9 andfurther comprising a distributed Bragg reflector structure for providingthe necessary feedback of the signal wave back into the waveguide lasercavity with the fabrication of the distributed Bragg reflector structurefollowing from standard etching procedures forming shallow surfacecorrugations on the surface of the channel waveguides.
 21. A method offorming a z-propagating waveguide, the method comprising:selecting anx-cut or y-cut LiNbO₃ sample; depositing a rare-earth material on theLiNbO₃ sample; incorporating ions of the rare earth material into thesample; delineating metal channels on the LiNbO₃ sample; andincorporating the metal channels into the LiNbO₃ sample.
 22. The methodof claim 21 and further comprising:positioning the LiNbO₃ sample on a Ptpad; positioning the pad on an alumina pedestal; positioning the aluminapedestal in a furnace; subjecting the LiNbO₃ sample, pad, and pedestalto a flowing oxygen atmosphere; and heating the alumina pedestal, Ptpad, LiNbO₃ sample to a temperature in the range of approximately 1000°C. to 1100° C. for predetermined period of time.
 23. The method of claim21 wherein the rare-earth ions are selected from one or more of thegroup consisting of Er³⁺, Nd.sup.³⁺, Yb³⁺, and Tm³⁺.
 24. The method ofclaim 21 wherein the rare-earth ions are thermally indiffused into theLiNbO₃ substrate.
 25. The method of claim 21 wherein the rare-earth ionsare incorporated into the LiNbO₃ substrate during the growth of the bulkcrystal.
 26. The method of claim 21 wherein the metal channels aremetals which increase the refractive index and form a waveguide, themetals being selected from one or more of the group consisting of Ti,Zn, Ni, and Cu.
 27. The method of claim 21 wherein the metal channelsare thermally diffused into the LiNbO₃ substrate.